Acid rain, or acid deposition, is precipitation with an abnormally high level of acidity that directly interfaces with the global water cycle. Normal rain is slightly acidic, typically maintaining a pH of about 5.6 due to dissolved carbon dioxide forming carbonic acid. Acid rain, however, frequently registers a pH below 5.0, sometimes dropping to 4.2 or 4.4, representing a significant increase in hydrogen ion concentration. This phenomenon fundamentally alters the chemical quality of water as it moves through its atmospheric, surface, and subterranean phases, compromising water quality and the natural processes of the cycle.
How Pollutants Enter the Atmospheric Phase
Acid rain originates from the emission of sulfur dioxide (SO2) and nitrogen oxides (NOx). These pollutants are primarily released from human activities, such as burning fossil fuels for electricity generation, industrial manufacturing, and vehicle operation. For example, in the United States, power plants burning fossil fuels account for roughly two-thirds of the total SO2 and one-quarter of the NOx emissions.
Once these gases are airborne, they undergo complex chemical transformations facilitated by sunlight, oxygen, and water vapor. Sulfur dioxide is oxidized into sulfur trioxide, which then reacts rapidly with atmospheric water to form sulfuric acid (H2SO4). Similarly, nitrogen oxides react with other atmospheric components, such as hydroxyl radicals, to produce nitric acid (HNO3).
The strong acids are incorporated into the water cycle through two processes. Wet deposition occurs when the acids dissolve in cloud droplets, fog, snow, or rain, falling as acidic precipitation. Dry deposition involves acidic particles and gases settling directly onto surfaces like soil or water bodies. Accumulated dry deposition is often washed off by the next rain event, re-entering the water cycle as an acidic solution.
Altering the Chemistry of Surface and Ground Water
When acidic precipitation reaches the surface, it immediately lowers the pH of runoff, rivers, and lakes. Acidification is pronounced in water systems lacking a strong buffering capacity, such as those situated on granite or non-calcareous bedrock, which cannot neutralize the incoming acid. Conversely, areas rich in base cations like calcium and magnesium (e.g., limestone regions) are more resistant because they can absorb and counteract the acid.
A significant consequence of this chemical alteration is the mobilization of toxic substances from the soil. Hydrogen ions in the acid rain displace essential plant nutrients, such as calcium and magnesium, from their binding sites in the soil. The loss of these buffering nutrients further weakens the soil’s ability to resist future acidification.
The increased acidity also converts naturally occurring, harmless aluminum compounds in the soil into a soluble, toxic form (Al3+). This dissolved aluminum is leached into surface water and groundwater, where it can harm aquatic life. Through infiltration, this acidic water penetrates the ground, changing the chemical composition of groundwater and potentially affecting adjacent regions as it feeds into surface water bodies.
Indirect Effects on Water Movement and Return
Beyond direct chemical changes, acid rain indirectly controls water movement and return to the atmosphere. Negative effects on vegetation, such as forests and crops, lead to decreased rates of transpiration. Transpiration is the process by which plants release water vapor from their leaves, representing a major pathway for moisture return in the water cycle.
Damage to leaves and needles, often resulting in reduced stomatal conductance, directly inhibits the plant’s ability to transpire water vapor. This reduced rate can locally decrease the moisture returned to the atmosphere, potentially altering regional moisture recycling. Acid deposition also depletes the soil of nutrients and releases heavy metals, weakening overall vegetation health.
The degradation of soil chemistry also impacts the physical movement of water across the land. As acid rain leaches nutrients and reduces soil health, the soil’s structure and composition are compromised. This reduces the soil’s capacity to absorb rainwater, leading to increased surface runoff. Increased runoff accelerates water movement away from the area, reducing time for infiltration and local evaporation, which alters the regional water balance.